A proposal has been made to develop devices that would generate spin-polarized electron currents characterized by polarization ratios having magnitudes in excess of 1. Heretofore, such devices (denoted, variously, as spin injectors, spin polarizers, and spin filters) have typically offered polarization ratios having magnitudes in the approximate range of 0.01 to 0.1. The proposed devices could be useful as efficient sources of spin-polarized electron currents for research on spintronics and development of practical spintronic devices.
The polarization ratio in question — denoted the current spin polarization — is a standard measure of efficiency of a spin-polarizing device. It is defined in terms of current densities along a given coordinate axis, by means of the following equation:
PJ> = (J↑ – J↓)/(J↑ + J↓),
where J↑ is current density of electrons in the “up” spin state and J↓ is the current density of electrons in the “down” spin state. If J↑ and J↓ can be made to have opposite signs — in other words, if electrons in opposite spin states can be made to move in opposite directions — then, as desired, it is possible to obtain |PJ|>1. By making |PJ|>1, one would make it possible to obtain a net spin flux with little net electric current.
One possible structure of a device according to the present proposal would be similar to the a-RITD structure described previously: The device would comprise an asymmetric composite InAs-GaSb well, sandwiched between AlSb barriers and degenerately-n-doped InAs emitter and collector electrodes (see figure). Unpolarized electrons from the conduction band of the InAs emitter electrode would tunnel through one AlSb barrier and travel through an asymmetric InAs-GaSb quantum well, where Rashba spin splitting would occur; they would then tunnel through the other AlSb barrier into the conduction band of the InAs collector electrode. The device would be operated in an intraband-tunneling regime, in which no bias would be applied through the thickness of the stack of layers. A lateral electric (an electric field parallel to the planes of the layers) would be applied to emitter layer. With appropriately chosen thicknesses of layers and an appropriate value of the applied lateral electric field, it should be possible to achieve |PJ|>1.
This work was done by David Z. Ting of Caltech for NASA’s Jet Propulsion Laboratory. NPO-30670
This Brief includes a Technical Support Package (TSP).
Electron-Spin Filters Would Offer Spin Polarization >1
(reference NPO-30670) is currently available for download from the TSP library.
Don't have an account?
Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory (JPL) that discusses advancements in spintronics, specifically focusing on electron-spin filters capable of achieving a current spin polarization greater than one (P_J > 1). Spintronics, or spin-based electronics, is a rapidly evolving field that aims to utilize the intrinsic spin of electrons, in addition to their charge, to enhance electronic device performance.
The report outlines a device concept involving an asymmetric resonant interband tunneling diode (a-RITD) designed to function as a spin filter. This device structure is characterized by a specific layer sequence that includes degenerately doped n-type InAs for the emitter and collector electrodes. Notably, the device operates in a zero-bias mode, applying a lateral electric field only in the emitter, which facilitates the transmission of unpolarized electrons through resonant tunneling while aligning their spins with quasibound states.
The primary goal of the research is to develop efficient spin injectors, polarizers, and filters that can significantly enhance the current spin polarization, which is a critical parameter in the performance of spintronic devices. The document emphasizes the importance of achieving high spin polarization for various applications, including data storage, quantum computing, and advanced electronic devices.
The report also references a related publication in the Physical Review Letters that discusses the Rashba effect, which is a key phenomenon in spintronics that can be exploited in the design of spin-filter devices. The Rashba effect arises from the spin-orbit coupling in systems lacking inversion symmetry, leading to spin-split energy bands that can be utilized for spin manipulation.
Overall, the document serves as a comprehensive overview of the current state of research in spintronics, highlighting the potential of electron-spin filters to revolutionize electronic devices by enabling greater control over electron spin. It underscores NASA's commitment to advancing aerospace-related technologies with broader implications for scientific and commercial applications. The report concludes by inviting further inquiries and collaboration through the Innovative Technology Assets Management office at JPL.
